1. Field of the Invention
The present invention relates to an optical semiconductor device including a reflector. More particularly, it relates to an optical semiconductor device used for optical information processing such as writing information onto and reading information from optical disks.
2. Description of the Related Art
FIGS. 12A and 12B are a sectional view and a top view, respectively, of an optical semiconductor device of the conventional configuration described in IEEE Transactions on Components, "Packing and Manufacturing Technology" Part B, Vol.18, No.2, May 1995, pp.245-249. In the drawing, numeral 2 denotes a laser diode (LD) or light emitting device, 4 denotes a reflecting surface, 5 denotes a photodiode (PD) or light detecting device, 61 denotes an Si substrate and 66 denotes a groove.
In such an optical semiconductor device, light emitted from the light emitting element 2, mounted on the Si substrate 61, in a direction parallel to the surface of the Si substrate 61 is reflected by the reflecting surface 4 in a direction perpendicular to the surface of the Si substrate 61. The reflected light is directed onto an optical disk or the like (not shown), is then split by a hologram, and detected as a reflected optical signal by the light detecting element 5 on the Si substrate 61.
Thus, the light reflected by the reflecting surface 4 must be directed exactly at right angles to the Si substrate 61. Therefore, conventionally, the Si substrate 61 has been subjected to etching with KOH or the like and the Si substrate has a surface deviating from a (100) plane of the monocrystalline Si substrate by an angle, referred to here as an off-angle, for example, by 9.7.degree.. The etching forms the groove 66 having a side face lying in a (111) plane of the monocrystalline Si substrate 61. The (111) plane forms an angle of 45.degree. with the surface of the Si substrate 61 so the (111) plane is used as the reflecting surface 4. Thus, the light emitted from the light emitting element 2, mounted on an etched bottom face, of the groove 66 parallel to the surface of the Si substrate, is reflected by the reflecting surface 4 in a direction perpendicular to the surface of the Si substrate.
However, since the light detecting device 5 is present in the Si substrate 61 before the groove 66 is formed by etching, it is necessary to protect the light detecting device 5 during the etching to form the groove 66. When SiO.sub.2 is used as a protective film that has a high resistance to etching by KOH or the like, the SiO.sub.2 film cannot be distinguished from an SiO.sub.2 passivation film on the light detecting device that has previously been formed on the Si substrate 61. Thus, it is difficult to selectively remove only the protective film after forming the groove 66 by etching.
A heat treatment step in forming the light detecting element 5 is usually carried out at a temperature of around 1000.degree. C. However, keeping the Si substrate 61 at such a high temperature causes crystalline defects to segregate, eventually resulting in a problem, for example, the reflecting surface 4 formed by etching may become rough.
Moreover, in photolithography, after forming the groove 66, the step in the groove 66 prevents a resist film on the Si substrate 61 from being planar, decreasing accuracy in forming the resist pattern.
To counter such problems, Japanese Patent Kokai Publication No. 9-64478 discloses an optical semiconductor device with a through hole in an Si substrate. A laser chip generating heat is mounted on a sub-mount fixed on a heat dissipation plate in the through hole. Thereby, the heat dissipation performance is improved. Light emitted by the laser chip is reflected in a direction perpendicular to the Si substrate surface by a mirror specifically installed on the Si substrate, not by a reflector formed by etching the Si substrate. However, such a configuration requires making the through hole in the Si substrate and installing the sub-mount, making production more complex, and also requires the laser chip to be fixed on the sub-mount with high accuracy, making the configuration unsuitable for mass production. Moreover, because the configuration has the through hole in the Si substrate, it is impossible to employ photolithography in which the Si substrate surface is coated with a resist film.
The present inventors have found that desirable light emitting characteristics can be obtained from a light emitting element having a low power output, up to 10 mW, or having a characteristic temperature, T.sub.0, of 90.degree.K or higher, even when the light emitting element is mounted directly on the Si substrate and also found that, by mounting a light emitting element and a reflector formed separately from the Si substrate on the Si substrate, it is possible to form an optical semiconductor device in which light emitted by the light emitting element is reflected in a direction perpendicular to the Si substrate surface, without etching the Si substrate with the light emitting element already mounted on the substrate as in the prior art.
FIG. 13 is a sectional view of an another optical semiconductor device of the conventional configuration. The light detecting diode 42 for reprocessing, tracking, or focusing signals or the like, and the reflecting surface 43 inclined at an angle of 45.degree. from the surface of the Si device substrate 100 are fabricated in the Si device substrate. The light emitting element 44, a source of light, is mounted on the Si device substrate 100, and the Si device substrate 100 is fixed in the package 45.
In such an optical semiconductor device, light emitted from the light emitting element 44 is reflected by the reflecting surface 43 in a direction perpendicular to the surface of the Si device substrate 100, and the reflected light is directed onto an optical disk or the like (not shown) through the hologram 46 and a lens or the like. Then the light reflected from the optical disk is split by the hologram 46, and directed onto the light detecting element 42 for reprocessing, tracking, or focusing the light, and recognized as a signal.
A uniform distribution of the light intensity on the reflecting surface 43 is needed in order to prevent an error in processing the light signal. Therefore, flatness of the reflecting surface is needed. Namely, the roughness of the reflecting surface 43 should be about 1% of the wavelength (for example, 650 nm for red light).
However, it is difficult to control the roughness of the reflecting surface 43 to about 1% of the wavelength using wet etching. Particularly, when the Si device substrate 100 has defects or the like in it, the roughness of the reflecting surface is increased because of the defects.
To counter such problems, Japanese Patent Kokai Publication No. 9-64478 discloses a optical semiconductor device with a Si reflector which has a reflecting surface fabricated by mechanical polishing and having an angle of 45.degree. from the bottom face. However it is difficult to fabricate such a reflecting surface having an angle of exactly 45.degree. from the bottom face by mechanical polishing, and it is difficult to fabricate such an optical semiconductor in mass-production.